Doppler system for speed and drift measurements



J. W. GRAY Oct. 7, 1958 DOPPLER SYSTEM FOR SPEED AND DRIFT MEASUREMENTSFiled Feb. 7, 1955 2 Sheets-Sheet 1 wmv J. W. GRAY Oct. 7, 1958 DOPPLERSYSTEM FOR SPEED AND DRIFT MEASUREMENTS Filed Feb. 7, 1955v 2Sheets-Sheet 2 l l fad-ifs .im 1LA INVENToR.

United States Patent DOPPLER SYSTEM FR SPEED AND DRIFT WASUREMENTSApplication February 7, 1955, Serial No. 486,435

Claims. (Cl. 343-8) This invention relates to a Doppler system florspeed and drift measurement. More specifically, the mvention relates tosuch a system that is entirely self contained and y is employed on amoving vehicle.

In general, this system employs at least two separate radio beamsradiated toward the earth and secures from them two radio echoes havingin general frequencies different from the transmitted frequency becauseof Doppler effect and different from each other if not alignedtransversely to the track. From these signals the so-calledDoppler-frequency voltages are derived. By addition of signalsrepresenting these voltages a signal representing vehicle speed issecured, and by subtraction there is secured a signal representing thegeometric relation of the radio beams to the ground track of thevehicle. A control circuit is energized by this subtraction signal toorient the radiators symmetrically with the ground track so that bymeasurement of the divergence of their position from the vehicularheading direction the drift angle is secured.

The purpose then of this invention is to provide an instrument formeasurement of the speed and drift of a Vehicle relative to the earth,this instrument including radio components for producing two or morebeams of radio energy and for receiving earth echoes of such beams. Theinstrument combines signals representing two or more of these echoes,the sum or averaged combination representing the vehicular speed and thedierence representing the beam and ground track geometrical relation.l

A further purpose of this invention is to employ the difference signalto orient the radiators symmetrically relative to the ground track,whereby the drift angle can be measured.

Further understanding of this invention may be secured from the detaileddescription and drawing, in which:

Figure 1 is a schematic representation of an embodiment of theinvention.

Figure 2 is a schematic diagram of the rate servo-mechanism used inconnection with the embodiment of Fig. 1.

Figure 3 depicts another embodiment of the speedmeasuring portion of thesystem.

Referring now to Figure 1, the essential components of the system areindicated generally as being used on an airplane. The system includesradio and computing components, the radio components employingfrequencies in the radar range, such as X-band microwave frequencies,and including several antennas arranged to radiate toward the earth sothat components of their directions will be in the horizontal directionof the ground track and in the horizontal direction transverse to theground track. Either the Janus type of operation in Which a Doppler beatfrequency is secured by combining two-echoes, one received from agenerally forward and the other from a generally rearward direction, orthe coherent oscillator type, in which an echo is combined with theoutput of a standard oscillator to secure a Doppler frequency, or anyother system productive of Doppler outputs may be employed.

2,855,590 Patented Oct. 7, 1958 The two antennas 11 and 12 are indicatedas dipoles with dish reflectors but may be of any directive type.A Theseantennas are iixed in a rigid but rotatable frame 13 so as to directtheir beamson either side of the horizontal ground track directionindicated by. the arrow 14. They also are pointed downward and forwardso that their beams strike the earth. The same antennas are employed forreception of the echoes of their respective beams.

The radio equipment includes, besides the antennas, a microwave pulsedradar transmitter 16, and right and left channel receivers 17 and 18associated respectively with theright antenna 11K and the left antenna12. These receivers include microwave guide conductors, duplexingequipment, a coherent oscillator, mixers, amplifiers, and demodulatorsVto produce at the outputs 19 and 21 alternating current voltages havingfrequencies in the audio range. These frequencies respectively representthe right and left Doppler frequencies, and together contain both speedy and drift information.

Specific systems meetingy the radio requirements of this invention aredescribed 4in patent applicationsl Serial No. 749,184, filed May 20,1947, by William J. Tull et al.; Serial No. 49,926, filed September 18,1948, by France B. Berger et al.; and Serial No. 410,882, filed Februaryl7, 1954, by John W. Gray et al., and hence further elaboration isunwarranted.

The Doppler frequency signals produced atconductors 19 and 21 areapplied to right and left-frequency trackers. These devices are similarand have as their function the production of a signal whose magnitude isrepresentative of the median Doppler frequency. The` frequency trackersalso lock to the Doppler energy input so that during changes in inputfrequency the median frequency of the Doppler spectrum is accuratelyfollowed.l Each frequency tracker includes as principal components amodulator 22v and 22A, an oscillator 23 and 23A, an` automatic gaincontrol amplifier 24 and 24A, a discrirninator 26 and 26A, and anintegrating amplifier 27 and 27A.4 Since both channels are identical indesign and operation only the operation of the right channel will bedescribed;VA

The audio frequency signals having a range of 1,000 to 16,000 cycles persecond are applied through conductor 19 to the modulator 22. Thesesignals, in general are c'ontained in a pulsed spectrum having a widthof about tenI percent of its center frequency, and the spectrumfrequency may change within its range in either direction at anytime.

The modulator 22 has impressed thereon the output of the radar receiverand the output of itsA associated local oscillator 23, producingtherefrom a mixed output signal having a frequency equal to thedifference of the oscillator and input Doppler frequencies. i Y

The oscillator 23 is of the multivibrator type producing a rectangularoutput voltage waveform, and is adjustable in frequency of outputbetween 21 and 36 kilocycles per second linearly proportional to the`magnitude, of a direct current control potential applied thereto. Inthe automatic operation of thel frequency tracker this oseillator ismaintained in such adjustment that the difference between its outputfrequency and the input signal spectrum median frequency is alwaysnominally 20 kc. p: s.

The output energy of modulator 22 is applied to the amplier 24 havingautomatic gain control. AThe ampliiied output of amplifier 24 is appliedto a discriminatoi 26 composed of two parallel channels, one tuned by asharply ldiscriminating filter to a selected frequency below that ofthenominal frequency of 20 kc. p. s., and the other tuned to a selectedfrequency above the nominal frequency of 20 kc. p. s. The outputamplitudes are subtracted to provide a direct ycurrent voltagerepresenting by its departure from a normal level the divergence of thediscriminator input from an exact 20 kc. p. s. frequency.

The output voltage of the discriminator 26 is applied to an integratingamplifier 27. This amplifier, in conjunction with a motor 28, capacitor29 and voltage divider 31 integrates the discriminator output to producea direct current potential at slider 32 representing by its magnitudethe Doppler frequency. This potential is applied to control thefrequency of local'oscillator 23, thesense of control being such as tomake the oscillator frequency follow the input signal median frequencyby a spaced frequency difference, however the signal median frequencymay fluctuate.

Thus the frequency tracker is made to follow fluctuationsin. thefrequency of its input signals and to reproduce them as direct currentvoltage fluctuations the integrated` values of which accuratelyrepresent the input frequency values. A frequency tracker of thischaracter constitutes the subject matter of the copending applicationSerial No. 314,306, filed October l1, l952, of John W. Gray et al., andhence further and detailed descripe tion is unnecessary.

Motor 28 operates a second voltage divider 33 which is energized by a400 C. P.- S. source represented by terminal 34. The operation of thisvoltage divider together withvoltage divider 31 by the same motor 28 isso arrangedk that the voltages of their sliders 32 and 36 areyalwayslinearly proportional, that of the latter thus also representingby its magnitude the Doppler channel' frequency.r This alternatingvoltage of voltage divider slider 36 and the similar voltage at slider37 of thek other channel are applied to an averaging device consistingof a center-tapped impedance 38. The center tap39 is connected to groundthrough a Calibrating voltage divider 41. The potential of slider 42 isamplifiedy ink amplifier 43 and is indicated in indicator 44. A tap 46is available for connection to a computer.`

4 shaft speeds are termed A and B and the spider shaft speed is termedC,

That is, the speed of output shaft S7 is the average of the input shaftspeeds. After suitable speed change in a gear 61 theoutput shaft speedis integrated in a revolution counter 62, the instantaneous indicationof which at any time therefore represents distance travelled by the thatthe spider output depends on the difference of the input speeds. Thatis, if D and E are input speeds,

2 in which F is the output'shaft speed. Since integration "of thisequation proves that the same relation holds for LThe voltmeterindicator 44 is calibrated in units of f airplane ground speed. It is`obvious that, when the antennas 11 and 12 are rotated about a verticalaxis so that their average horizontal direction of emission andreception is along the ground track, theDoppler frequencies derived fromtheir echo signals will be equal. When the local oscillators 23 and 23A,having tracked `these equal frequencies, emit equal frequencies they arecontrolled by equal voltages. To produce these equal voltages thediscriminators 26 and 26A have caused motors 28 and 28A to come to restat equal shaft positions. Thus the alternating voltages at sliders 36and 37 are equal to the average voltage applied through divider 41 andamplifier 43 to indicator 44. This indicated voltage represents thevalue of aircraft speed.

When the antennas are not accurately pointed, the voltages at sliderers36 and 37 are not equal but their average at junction 39 stillrepresents aircraft speed with but small error. For example, if theantennas are off the ground track by 5 the error in speed indication isless than 0.5%.

The alternating voltage outputs of sliders 36 and 37 are also applied totwo rate servoamplitiers 47 and 47A. Servoamplifier 47 is connected tooperate a motor 48 which through shaft 49 operates a feedback generator51, thus constituting a servomechanism rotating the shaft 49 at a rateor speed representing the channel Doppler frequency. The otherservoamplifier is similarly connected to a motor 52, shaft 53 andgenerator 54, the shaft 53 rotating at a speed representing the otherchannel Doppler frequency.

The speeds of shafts 49 and S3 are added in a differential gear 56, sothat the output shaft 57 speed represents `the average motor shaft speedand therefore the airangular displacements, the output angulardisplacement is one-half of the difference of the input angular shaftdisplacements, and represents the angle by ,which rthe average antennadirection departs from the ground track horizontal direction.

The shaft 64 is connected to the slider 68 of a voltage divider 69energized by a source 71 of 400 C. P. S. potential at its terminals 72and 73, in which the output mechanical deflection is converted into aproportional electrical potential. The slider 68 is connected to anazimuth position servomechanism consisting of amplifier 74, motor 7-6,and voltage divider 77. The position of slider 78 then represents theposition of slider 68. The slider 78fis connected to the amplifier 74through conductor 79 to secure servomechanism feedback operation.

Shaft 81 of motor 76 is also connected to rotate the antenna frame 13about its vertical axis 82, the sense of rotation relative to the errorsignal being such as to tend to reduce the error or difference signalmagnitude at shaft 64. Thus the entire loop including radio andcomputing components constitutes a mechanism for maintaining theantennas with their average horizontal component of beam directionaccurately pointed along the airplane horizontal ground track direction.

When wind having a component transverse to the ground track exerts forceon the airplane, the airplane heading and ground track horizontalcomponents differ by an angle termed the drift angle. This angle istaken `off at the pivot 82 of the antennas and is indicated by indicator83. Conductor 84 is provided for computer connection when required.

The two rate servo controls 47 and 47A are similar and their circuitsare shown in Fig. 2, which is specifi- 4cally the circuit of the rightrate servo control 47, its

motor 48 and its generator 51, which together form a linear rateservomechanism.

The input signal impressed on the conductor 86, Figs. l and 2,consisting of a 400 C. P. S. voltage representing by its magnitude theDoppler frequency in the right channel, is applied through an isolatingresistor 87, Fig. 2, and coupling condenser 88 to a triode alternatingcurrent amplifier 89. The output is coupled through condenser 91 to onecontrol grid 92 of a balanced stage comprising triodes 93 and 94. Theother control grid 96 is grounded and the tubesare coupled through acommon cathode resistor 97 to a source `of negative potential.

`Pour hundred cycle power is also applied to the cathodes Xcept duringthe lmost negative part of the cycle. When this part of the cycle occursthe tubes conduct differentially in accordance with whatever signalpotential exists at that time on grid 92. The two tubes therefore sensephase as well as amplify. They conduct in pulses which are smoothed tocontinuous potential by condensers 101, 102, 103 and resistor 104, thesense and magnitude depending on the phase and magnitude of the inputsignal. This stage is thus phase sensitive and although it has twoalternating current inputs, has a direct-current output. The networksincluding resistors 106 and 107 and condensers 108 and 109 permit rapidtransmission of changes in signal magnitude.

The potential between conductors 111 and 112 is applied to adirect-coupled amplier comprising triodes 113 and 114, which iu turncontrols a magnetic amplifier comprising saturable transformers 116 and117. The primary windings 118 and 119 are connected to a source of400-cycle potential, so that the secondary output atl conductors 121 and122 is 400 C. P. S. alternating potential having sense and magnituderepresenting that applied to the first tube 89. This output potential isapplied to the servo motor 48 driving the 400 C. P. S. tachometergenerator 51. A feedback connection 123 is made from the generator 51through an isolating resistor 124 to the junction of isolating resistor87 and coupling condenser 88. Polar-ities are so arranged that thefeedback phase through resistor 124 is opposite to that of the signalapplied through resistor 87. Therefore when these potentials are equal,they cancel and when unequal the phase applied to triode 89 depends onwhich is larger. This circuit is therefore highly sensitive and rapidlyresponsive to changes in signal, and is also linear.

Under some circumstances the output voltages of voltage divider sliders36 and 37, Fig. 1, are not as suitable for indication of speed as arethe output voltages of the feedback tachometers or generators 51 and 54,which voltages are representative of the speeds of shafts 49 and 53, theaverage of these speeds and of these voltages representing aircraftspeed.

The use of these generator outputs for this purpose is depicted in Fig.3. Voltage divider sliders 36 and 37 are connected to apply theirpotentials as inputs to rate amplifiers 47 and 47A, which energizemotors 48 and 52 as before described. Feedback generators 5 1 and 54have their output conductors 126 and 127 connected to the ends of acenter-tapped impedance 128. A voltage divider 129 connects the centertap 131 to ground. The voltage divider slider 132 is connected throughan amplifier 133 to a voltmeter indicator 134 calibrated in aircraftground speed units.

The generator output voltages are thus averaged by the impedance 128,and the voltage impressed across voltage divider 129 is the average ofthe generator outputs. A portion of this average voltage is imposed onslider 132, is amplified by amplifier 133 and is indicated by indicator134. The adjustment of the slider 132 constitutes a calibrationadjustment for the system.

What is claimed is:

l. A Doppler system of the class described comprising, means mounted ona craft for radiating signals toward the earths surface, means on saidcraft for receiving signals reflected from the earths surface in a firstdirection, means mounted on said craft for receiving signals reflectedfrom the earths surface in a second direction, said reflected signalsbeing shifted in frequency relative to the frequency of said radiatedsignals in proportion to the components of velocity of said craft insaid respective directions, means'for converting the shift in frequencyof said first mentioned reflected signal into a first alternatingvoltage amplitude, means for converting the shift in frequency of saidsecond mentioned reflected signal into a second alternating voltageamplitude, and means responsive to the average amplitude of said firstand sec- 6 ond alternating voltage amplitudes for indicating aircraftspeed.

2. A doppler system of the class described comprising, means mounted ona craft for 'radiating signals toward the earths surface, means on saidcraft for receiving signals reflected from the earths surface in a firstdirection, means mounted on said craft for receiving signals reflectedfrom the earths surface in a second direction, said reflected signalsbeing shifted in frequency relative to the frequency of said radiatedsignals in proportion to the components of velocity of said craft insaid respective directions, means converting the shift in frequency ofsaid first-mentioned reflected signal into a first shaft rotation, therotational speed of which is proportional to said frequency shift, meansfor converting said second-mentioned reflected signal into a secondshaft rotation, ythe rotational speed of which is proportional to thefrequency shift of the second-mentioned reflected signal, a subtractingdifferential gear operated by said first and second shaft rotations toproduce an output shaft rotation which is the difference of said firstand second rotations, means operated by said output shaft for orientingsaid means for receiving signals reflected in first and seconddirections of reception relative to the ground track of said vehicle,means operated by said output shaft for indicating a shaft anglerepresenting aircraft drift angle, means converting the shift infrequency of said first-mentioned reflected signal into a firstelectrical representation, means for converting the shift in frequencyof said second-mentioned reflected signal into a second electricalrepresentation, and means responsive to the average of said first andsecond electrical representations for indieating aircraft speed.

3. A Doppler system in accordance with claim 2 in which said first andsecond electrical representations are first and second alternatingvoltage amplitudes.

4. A doppler system of the class described comprising, means mounted ona craft for radiating signals toward the'earths surface, means on saidcraft for receiving signals reflected from the earths surface in a firstdirection, means mounted on said craft for receiving signals reflectedfrom the earths surface in a second direction, said reflected signalsbeing shifted in frequency relative to the frequency of said radiatedsignals in proportion to the components of velocity of said craft insaid respective direction, means for converting the shift in frequency'of said first mentioned reflected signal into a first shaft rotation thespeed of which is proportional to said frequency shift, means forconverting said second mentioned reflected signal into a second shaftrotation the speed of which is proportional to said frequency shift,asubtracting differential gear operated by said first and second shaftrotations producing as an output a shaft rotation which is thedifference of said first and second shaft rotations, and means operatedby said output Shaft rotation for orienting said means for receivingsignals reflected in first and second directions of reception relativeto the ground track of said craft.

5. A Doppler system of the class described comprising, a rotatableantenna assembly mounted on a craft, said assembly including a pair ofantennas for radiating signals toward the earths surface and forreceiving signals reflected therefrom, said antennas being directed indifferent directions relative to the ground track of the craft, therespective reflected signals received by said antennas being Dopplershifted in frequency in proportion to the component of velocity of thecraft in the respective directions of reception, means for convertingthe Dopplershift in frequency of the signal received by one of said pair-of antennas into a first shaft rotation the speed of rotation of whichis proportional to said Doppler shift, means for converting the Dopplershift in frequency of the signal received by the other of said pair ofantennas into a second shaft rotation the speed of rotation of which isproportional to said last mentioned Doppler shift, a

gaat f f 'y `sulltracting differential gear operated by' Vsaid irst yandksecond shaft yrotations producing therefrom an output 1 rshaftArotation; which listhe difference yof ysaidfirst and scocnd shaftrotations, a position servo, lmeans for ener- L gizing said servo inaccordance with the rotationof said loutput-shaft tofproduce an outputshaft angular deftection representing craft drift angle, `andlmeansoperated by the output of,y ysaid position servo for rotatingsaidantennaf assembly.

6, A Doppler systemgof the class described comprising,

yafrotatable antenna assembly mounted on a craft, said 'assemblylincluding Aa ypair of antennas xedwith respect ing currentpotentialsyrotations, of said rst and second shaftshaving'speeds'proportionaltothe'potentialmagnitudes, a subtractingdifferential operatedby said shaft f lrotations producing therefrom a'diifercnce rotation, and `means:forrotating.said antenna assemblyinaccordancey thereto `for radiating signals toward 'the' earthslsurfacey and for-,receiving signals reflected therefrom, said anmeansfor converting the Dopplerkshift in frequency of `the signal receivedby'one'of said pair ofantennas into arotationof a rst shaft the 'speed 'oflrotation of which v is proportional to said Doppler shift,- rneans forconverty 'ing' the Doppler shift in' frequencyfof the ysignal receivedby the other of said pair of antennas'into a yrotation of a.',ksecondshaft the speedofrotation-of which is propor- 1 tionalk toysaid. last mentioned Dopplery shift, a ysubtracting differential gear ioperated by ysaid jfirst and .second i n shaft ,rotations producingtherefrom a difference shaft rotation, a.r positionr servo, means foryenergizing said servo in accordance with said' .differencershaft-rotation;

n means operated by the output ofisaid position servo for y"rotatingsaid antenna assembly,k indicator means, and

- means energizing saidindica'tor means fromsaid first and second shaftsin proportion to the average` sum of said yrstr and ysecond shaftrotations.

with said difference rotation.

l0. A Doppler system as set forth in claim 9 including arst voltagegenerator operated by said first-shaft, a secondvoltage generatoroperated by said second'shaft," l() means connected to ysaid generators,for averaging the speed magnitudes g of said .first and second s shafts,dicator energized by said last-named means.

; f rll. kA Doppler ysystem of the class described comprising, arotatable antenna yassembly mounted .on la craft said antenna assemblyincluding a pair of antennas fixed with respectl thereto for radiatingsignals toward the eartl-tsy surfaceA and forreceiving vsignals reectedthere- L fronnsaid antennas being. directed indifferentk directionslrelative to the ground track of the craft, a pair of receivingchannels'each of which is yseparately energizedby a respective one ofsaid pair of antennas; .each of `said`r receiving channels, including anoscillator the frequency i of which is 'controlled in' accordance with.the magnitude signal of the channel and the youtput-of the oscillatoryimy y pressed thereon andk producing a beat freq'ue'nczyr signalv ftherefrom, means in each said channel energized by said i* beatfrequencyfor generating a; voltage the magnitude ofy y which isproportional tothe departure ofL saidy beaty fre-y f of a direct current potentialimposed thereon, a :modulat y tor yin yeach said channel havingtherespective received quency fromay selectedrfixed frequency, means ineach 7. kAt Doppler system as set forth in clairn inwhich ond shaftrotations.

8. ADoppler system as set forth in claim 7 including a lirst voltagegenerator operated by said first shaft rotation producing a voltagewhich is proportional to the rotational speed thereof, a second voltagegenerator operated by said second shaft rotation producing a voltagewhich is proportional to the rotational speed thereof, means foraveraging the voltages generated by said first and second generators,and an indicator energized by said average voltage.

9. A Doppler system of the class described comprising, a rotatableantenna assembly mounted on a craft said antenna assembly including apair of antennas fixed with respect thereto for radiating signals towardthe earths surface and for receiving signals reflected therefrom, saidantennas being directed in different directions relative to the groundtrack of the craft, a pair of receiving channels each of which isseparately energized by a respective one of said pair of antennas; eachof said receiving channels including an oscillator the frequency ofwhich is controlled in accordance with the magnitude of a direct currentpotential imposed thereon, a modulator in each said channel having therespective received signal of the channel and the output of theoscillator impressed thereon and producing a beat frequency signaltherefrom,

means in cach said channel energized by said beat frequency forgenerating a voltage the magnitude of which .is proportional to thedeparture of said beat frequency from a selected fixed frequency, meansin each channel for integrating said voltage t-o produce said directcurrent potential the amplitude thereof being proportional to Vthefrequency of the respective received signal, means in each channel forgenerating an alternating current potential proportional to saidrespective direct current potential, first and second shafts, means ineach of said pair of channels for generating from said respectivealternatthe means for energizing said indicator means includes a f'summation gear differential operatedby said first and Secchannelforintegrating. said voltage to produce said direct y current potentialkthe K4amplitude thereofk being` 'proportional to they frequency of therespective received signal,

meansin eachy channel for generating an alternating current potentialproportional tosaid respective direct current potential, rst and secondshafts, means in each of :said pair of channels for generating from saidrrespective y alternating current potentials rotations of lsaid kfirst:and second shafts having speeds proportional to the potentialmagnitudes, an adding differential operated by said first and secondshaft rotations producing therefrom a shaft rotation having the averagespeed of said shaft rotations, and a revolution counter operated by saidadding differential, said revolution counter indication representingelapsed total craft distance travelled.

12. A Doppler system of the class described comprising, a rotatableantenna assembly mounted on a craft said antenna assembly including apair of antennas fixed with respect thereto for radiating signals towardthe earths surface and for receiving signals reflected therefrom, saidantennas being directed in different directions relative to the groundtrack of the craft, a pair of receiving channels each of which isseparately energized by a respective one of said pair of antennas; eachof said receiving channels including an oscillator the frequency ofwhich is controlled in accordance with the magnitude of a direct currentpotential imposed thereon, a modulator in each said channel having therespective received signal of the channel and the output of theoscillator impressed thereon and producing a beat frequency signaltherefrom, means in each said channel energized by said beat frequencyfor generating a voltage the magnitude of which is proportional to thedeparture of said beat frequency from a selected fixed frequency, meansin each channel for integrating said voltage to produce said directcurrent potential the amplitude thereof being proportional to thefrequency of the respective received signal, means in each channel forgenerating an alternating current potential proportional to saidrespective direct current potential, passive means for generating fromthe two alternating current potential outputs of said last-named meansan alternating potential representative of the average thereof, andindicating means operated by said alternating potential for indicatingits magnitude representative of craft ground speed.

13. A Doppler system of the class described comprising, a rotatableantenna assembly mounted on a craft said antenna assembly including apair of antennas fixed with respect thereto for radiating signals towardthe earths lsurface and for receiving signals reflected therefrom, saidantennas being directed in different directions relative to the groundtrack of the craft, a pair of receiving channels each of which isseparately energized by a respective one of said pair of antennas, eachof said receiving channels including an oscillator the frequency ofwhich is controlled in accordance with the magnitude of a direct currentpotential imposed thereon, a modulator in each said channel having therespective received signal of the l channel and the output of theoscillator impressed thereon channel for generating an alternatingcurrent potentiall 4proportional to said respective direct currentpotential,

first and second shafts, means in each of said pair of channels forgenerating from said respective alternating current potentials rotationsof said first and second shafts having speeds proportional to thepotential magnitudes, a subtracting differential operated by said shaftrotations producing therefrom a difference rotation, means for rotatingsaid antenna assembly in drift angle in accordance with said differencerotation, passive means for generating from said alternating currentpotentials proportional to said respective direct current potentials analternating potential representative of the average thereof, andindicating means operated by said alternating potential for indicatingits magnitude representative of craft ground speed.

14. A Doppler system of the class described comprising, means mounted ona craft for radiating signals toward the earths surface, means on saidcraft .for receiving signals reflected from the earths surface in a rstdirection, means mounted on said craft for receiving signals reflectedfrom the earths surface in a second direction, said reflected signalsbeing shifted in frequency relative to the frequency of said radiatedsignals in proportion to the components of velocity of said craft insaid respective directions, means for converting the shift in frequencyof said rst mentioned reflected signal into a shaft rotation the speedof which is proportional to said frequency shift, means for convertingthe-shift in frequency of said second mentioned reflected signal into ashaft rotation the speed of which is proportional to said frequencyshift, and means operative by the average of said first and secondmentioned shaft rotations for indicating the speed of said craft.

15. A Doppler system of the class described comprising, means mounted ona craft for radiating signals toward the earths surface, means on saidcraft for receiving signals reflected from the earths surface in a rstdirection, means mounted on said craft for receiving signals reflectedfrom the earths surface in a second direction, said reflected signalsbeing shifted in` frequency relative to the frequency of said radiatedsignals in proportion to the components of velocity of said craft insaid respective directions, means converting the shift in frequency ofsaid first-mentioned reflected signal into a first shaft rotation, therotational speed of which is proportional to said frequency shift, meansfor converting said second-mentioned reflected signal into a secondshaft rotation, the rotational speed of which is proportional to thefrequency shift of the second-mentioned reflected signal, a subtractingdifferential gear operated by said first and second shaft rotations toproduce an output shaft rotation which is the difference of said firstand second rotations, means operated by said output shaft for orientingsaid means for receiving signals reflected in rst and second directionsof reception relative to the ground track of said vehicle, a craftground speed indicator, and means averaging the speeds of said rst andsecond shaft rotations to operate said indicator.

References Cited in the ille of this patent UNITED STATES PATENTS2,193,361 Rice Mar. 12, 1940 2,422,064 Anderson et al June 10. 19472,476,032 Feldman et al. July 12, 1949

